The signals produced by the detectors of an array of detectors, such as those of an infrared focal plane array, must generally be processed by signal processing electronics. Focal plane arrays may be used on spacecraft for detecting infrared (IR) or other radiation. The signal processing electronics may be implemented on a plurality of semiconductor chips that are stacked together and electrically connected along one edge to the detectors of the array.
In the three-dimensional (3-D) packaging of such electronics, the silicon die are stacked together vertically, and bonded together to form a module assembly. Such packaging necessitates the routing of conductors from the silicon face of the die to the edge of the die for placement of connections to the next assembly, such as the connector pads for the detectors. For example, the module may be connected to an infrared detector array. Such connections may be made through indium bumps on the backside of the detector array. Good electrical connections must be made between the detectors of the detector array and the connectors to the electronics of the silicon die. This requires that the connectors be accurately positioned, and that the plane containing the ends of the connectors be extremely smooth to accurately mate with the detector array. It is also important to maintain electrical isolation of the conductors from each other and from adjacent silicon die, and to electrically isolate the dies from each other.
In modern focal plane detector array technology, an objective is to increase the density of the detectors, which requires closer spacing of the conductors while maintaining electrical isolation. The centers of the detectors may be only 0.005 inches apart. The conductors used to contact the detectors and connect them to the die electronics are extremely narrow in width (perhaps 0.003 inches), and extremely thin (perhaps on the order of 0.00025 inches). Furthermore, the conductors or leads may be formed of extremely fine gold, which is a soft material.